IMPORTANT AND DETAILED CORRESPONDENCE BETWEEN TWO OF THE GIANTS OF 20TH CENTURY PARTICLE PHYSICS, written during an intensely creative period for both men. Rossi and Heisenberg met in early 1930, and Heisenberg was immediately intrigued with Rossi's experiments with cosmic rays, which was coming into its own as a central focus of particle physics, culminating in the Nobel Prize for Hess and Anderson in 1936 for their contributions. The correspondence here represents an important scientific dialogue on the front lines of experimental physics at a crucial time in the development of particle physics, taking place just before Heisenberg was awarded his own Nobel Prize in 1932. At the time, Rossi was just beginning a series of experiments with his newly developed coincidence circuit, and Heisenberg, motivated by recent developments in relativistic quantum theory (a field which he helped found), correctly recognized that the study of high-energy cosmic rays was indispensable to the observation and testing of high-energy quanta.Beginning with two letters from Rossi (dated September 13 and November 19, 1930), and continuing through 1932, the two friends working on the frontiers of quantum mechanics exchanged and shared their experimental progress. The correspondence is full of scientific detail, captured in this central letter from Heisenberg dated January 9, 1932: "Over the Christmas holidays I have had a chance to think about your work in great detail, and have discussed this with Bohr at length. We are now – under the condition that the experiments are correct – in full agreement with you. Either the radiation is a corpuscular (i.e. electron), in other words the Stoermer Theorie doesn't work, or that the primary radiation is absorbed much faster than it can be expected. The Klein-Nishina formula is probably wrong by a factor of 10."Prior to Rossi's experiments, almost nothing was known about the composition and strength of cosmic rays. Inspired by the 1929 Bothe-Kolhorster experiment (which recorded a cosmic-ray particle with 40 times the penetrating force of the electron), Rossi conceived and carried out a pioneering set of experiments that gave scientists their first real sense of the velocity, strength, and composition of cosmic rays. Rossi constructed two new types of tracking device for his experiment, each of which recorded triple coincidences of pulses using three Geiger counters separated by shielding blocks of lead, which subsequently became the standard measuring device for cosmic ray research. Rossi's experiments showed that primary cosmic ray particles are both capable of traversing great thicknesses of lead and also capable of precipitating multiple secondary particle showers. These results were so novel and unexpected – no known process at the time could explain the abundant production of secondary particles – Rossi could not find a publisher for his experimental reports until Heisenberg himself vouched for the truth of the reporting. The letters reflect in real time these experiments and developments, including two Heisenberg sketches of Rossi's famous circuit, including a proposed alteration.Lengthy, detailed correspondence pertaining directly to major scientific discovery is rarely found in private hands. The Heisenberg-Rossi dialogue contained here represents a remarkable archive – containing significant unexplored research potential – capturing the two great scientists as they explore and discuss ground-breaking experiments and theory at the birth of particle physics.

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